Transmission displays such as typically liquid-crystal panels comprise a backlight that emits light in area and a display panel that comprises an array of dot pixels, in which letters and images are displayed through modulation of the light transmittance of each pixel of the display panel. The backlight includes, for example, one which comprises a combination of a halogen lamp, a reflector and a lens capable of modulating the luminance distribution of the outgoing light; one which comprises a waveguide and fluorescent tubes as disposed adjacent to the side surfaces of the waveguide and in which the light from the fluorescent tubes enters the waveguide and goes out through the surface of the waveguide perpendicular to the side surfaces thereof; and one which comprises a waveguide and a fluorescent tube as disposed inside the waveguide (direct backlight). The backlight employing a halogen lamp is used essentially in liquid-crystal projectors that require high luminance. On the other hand, the backlight employing a waveguide can be thinned and is much used in direct-view displays for liquid-crystal TVs, personal computers, etc.
It is desired that backlights for use in liquid-crystal TVs, notebook-size personal computers and the like consume reduced power while producing high luminance. For this, increasing the number of light sources such as cold cathode fluorescent lamps in backlights could realize the increase in the luminance to be produced, which, however, is not practicable as resulting in the increase in the power consumption by backlights.
On the other hand, the range of viewing angles for liquid-crystal panels is extremely narrow. At high viewing angles, or that is, large angles from the direction normal to the surface of the display (that is, from the direction just in front of the display surface), liquid-crystal panels exhibit low contrast and even changes in visual chromaticity and produce impractical images. Specifically, in liquid-crystal panels, the light being outputted in the oblique direction relative to the display surface is not utilized for viewing the display surface, and the degree of light utilization in liquid-crystal displays is not always high.
To solve the problem in the art, a backlight structure has been developed, which comprises a slab waveguide and light sources such as cold cathode fluorescent lamps or the like as disposed adjacent to the both side surfaces of the slab waveguide and in which a light diffuser and a prism sheet are provided above the slab waveguide (see Japanese Patent Application Laid-Open (JP-A) Hei-6-3667 and Hei-6-67004). Apart from this, another technique has been proposed of using two prism sheets as overlapped with each other in such a manner that their patterns are perpendicular to each other for two-dimensionally modulating the light passing through them. The backlight structures of those types will be effective in increasing the luminance of the light being outputted in the direction normal to their front surface, but are still defective in that the light being outputted at angles in the oblique direction overstepping the direction normal to the light output surface of the backlight still has relatively high luminance (the light of this type is hereinunder referred to as a side peak). In order to reduce the side peak, the angles at which light is outputted must be broadened, which, however, inevitably results in the decrease in the luminance of the outgoing light.
JP-A Hei-8-221013 discloses a backlighting apparatus comprising a first waveguide 23 and a second waveguide 24 as disposed adjacent to the surface of the first waveguide 23, as in FIG. 16, in which the light from the light sources 21 having been reflected by the reflector 22 and entering into the first waveguide 23 through its side surfaces is transmitted toward the front surface of the display panel. In this backlighting apparatus, however, the light having entered the first waveguide 23 in the direction perpendicular to one side surface of the first waveguide 23 directly passes inside the first waveguide 23 toward the other side surface, without going toward the second waveguide 24. Therefore, the light availability through the backlighting apparatus is not high. In addition, the light being outputted by the backlighting apparatus is not polarized. Therefore, in the combination of the backlighting apparatus and the transmission liquid-crystal panel 25, as in FIG. 16, the component of light being polarized by the polarizer 26 as disposed on the light input surface of the panel 25, in the direction that is the same as the direction of the absorption axis of the polarizer 26 is absorbed by the polarizer 26, thereby resulting in that the luminance of the backlighting apparatus is decreased by 50%. For these reasons, the backlighting apparatus could not realize sufficient light availability.